1. Field of the Invention
This invention relates to a vertical cavity surface emitting laser and a method of manufacturing a two-dimensional photonic crystal of such a vertical cavity surface emitting laser.
2. Description of the Related Art
Known surface emitting lasers include vertical cavity surface emitting lasers prepared by sandwiching an active region at opposite sides thereof between two reflectors and forming an optical cavity in the direction perpendicular to the substrate surface so as to emit a light beam in the direction perpendicular to the substrate surface.
Research efforts are being intensively paid on vertical cavity surface emitting lasers because they provide a number of advantages as listed below.
A vertical cavity surface emitting laser emits a light beam having a profile close to a circle that can be optically coupled with an optical fiber with ease. The laser can be examined in a wafer without requiring cleavage. The laser can be driven to operate at a low threshold of about 0.1 milliamperes. The light beam emitted from the laser can be modulated at high speed. Such a vertical cavity surface emitting laser is particularly advantageous for in-plane integration.
While a vertical cavity surface emitting laser provides many advantages, it is accompanied by problems including that the diameter of the beam that can oscillate in a single transverse mode is small (not greater than 4 μmø) and that the polarization thereof is not stable. These and other problems make the laser less feasible for practical use.
Meanwhile, Appl. Phys. Lett., Vol. 80, 3901 describes an attempt of transverse mode control by arranging a two-dimensional photonic crystal in a distributed Bragg reflector (to be referred to as DBR hereinafter) at one side of a vertical cavity surface emitting laser.
According to the above article, a two-dimensional photonic crystal that includes a point defect portion at the center is prepared by boring holes in the DBR operating as the upper reflector of the vertical cavity surface emitting laser and confining light there by utilizing the difference of effective refractive index for the purpose of transverse mode control.
More specifically, the point defect portion is used as core and the photonic crystal part surrounding the core is operated as clad for the purpose of transverse mode control.
Appl. Phys. Lett., Vol. 82, 1344 reports that it is confirmed that the oscillation wavelengths of vertical cavity surface emitting lasers having a two-dimensional photonic crystal in the upper DBR differ from each other when the depths of their holes are differentiated. This is because the effective refractive index varies as a function of the depth of the holes.
Physical Review B, Vol. 65, 235112 and Optics Express, Vol. 13, 6564 report that it is found as a result of looking into reflected light and transmitted light of a two-dimensional photonic crystal slab that light of a certain frequency is reflected with an efficiency of about 100% when light is made to strike the two-dimensional photonic crystal slab in a direction perpendicular to the surface of the slab. Thus, a two-dimensional photonic crystal slab can be used as a reflector having a high reflectivity.